Supercapacitors, a class of electrical-energy-storage devices with high power density (103-104 W kg−1) and long cycling life (>100,000 cycles), hold great promise for a broad spectrum of applications, such as hybrid electric vehicles, power tools and mobile electronic devices. Generally, a supercapacitor may be operated based on the electrochemical double-layer capacitance (EDLC) formed along an electrode/electrolyte interface of an electrochemical cell. For example, a device is assembled by two electrodes made with the same carbon material and separated by a porous separator soaked in electrolyte. The charge (energy) is stored by separating electrolyte ions of opposing charges on the surface of porous carbon in the electrodes. Specifically, during the charge process, the positively charged ions will immigrate to a negative electrode and form a charged double layer; at the same time, the negative charged ions will accumulate on the surface of a positive electrode and form another double layer. The rapid charge/discharge process provides the capacitor with a high power density, yet the energy density is limited by its effective double-layer area. Therefore, a high surface-area carbon with optimal pore structure is highly desired for high performance device application.
However, current supercapacitors are still limited by low energy density, and improving the energy density while maintaining high power density is essential to realize such great potentials. Since supercapacitors generally rely on electrical double-layer capacitance or pseudo-capacitance, to realize high energy density requires a sufficient number of ions to be absorbed on or inserted into the electrodes, while realizing high power density requires rapid transport of ions and electrons between the electrodes. To satisfy these criteria, the electrode materials should exhibit high ion-storage density, excellent electronic conductivity, and effective ion-transport capability.
The current electrode materials for supercapacitors are mainly activated carbons, carbon nanotubes (CNTs) and metal oxides. In this context, activated carbons (AC) possess high surface area, high microporosity and moderate electronic conductivity. Capacitances up to 300 F g−1 in aqueous electrolyte or 120 F g−1 in organic electrolyte may be achieved at low discharge rates, which correspond to energy densities of ˜10 and 30 Wh kg−1, respectively. At high discharge rates, however, their storage performance radically deteriorates due to the lagged ion transport within their torturous microporous channels. Carbon nanotubes (CNTs), on the other hand, possess excellent electronic conductivity and readily accessible external surfaces that can provide outstanding rate performance. However, CNTs generally possess low surface areas, which provide low specific capacitances of less than 100 F g−1 in aqueous electrolyte or 50 F g−1 in organic electrolyte, respectively. Alternatively, metal oxides, such as RuO2, MnO2 and V2O5 may provide much higher pseudo-capacitance through faradic reactions. Except the cost-prohibitive RuO2, however, such materials are intrinsically poor ionic and electronic conductors, which limit their high-power application. Much effort has been devoted to making better electrode materials recently. For example, high-surface-area carbons with more regulated pore channels, such as carbide-derived carbon and zeolite-templated carbon, were synthesized with capacitances up to 150 F g−1 and improved high-rate performance in organic electrolytes, however, their synthesis is extremely ineffective. Similarly, surface functionalized CNTs may provide capacitances up to ˜150 F g−1 in H2SO4; however, such modified CNTs are easily degraded during cycling. Accordingly, making high-performance supercapacitor materials remains a challenge.